The present invention relates generally to a manner by which to facilitate recovery of digital data communicated to a receiver upon a communication channel susceptible to distortion. More particularly, the present invention relates to apparatus, and an associated method, by which to perform MLSE (maximum likelihood sequence estimation) upon digital data received at the receiver. Received data is first applied to a reliability indicia generator that operates at a low complexity via channel impulse response length reduction. Reliability information is so-derived at the reliability indicia generator with respect to possible values of the transmitted data. The reliability information is then used at a soft trellis decoder to reduce the matrix size of the trellis by removing out of the trellis nodes indicated by the reliability information unlikely to correspond to values of the transmitted digital data actually sent upon the communication channel.
A communication system provides for the communication of data between a sending station and a receiving station. The sending and receiving stations are interconnected by a communication channel. Data to be communicated by the sending station to the receiving station is converted, if necessary, into a form to permit its communication upon the communication channel.
The receiving station is positioned, or is otherwise operable, to detect the data transmitted upon the communication channel to the receiving station. The receiving station acts upon, if necessary, the received data to recreate the informational content of the data. In an ideal communication system, the data, when received at the receiving station, is identical to the data when transmitted by the sending station. However, in an actual communication system, the data is distorted during its communication upon the communication channel. Such distortion distorts values of the data when received at the receiving station. If the distortion is significant, the informational content of the data, as transmitted, cannot be recovered.
An exemplary type of communication system is a radio communication system. In a radio communication system, the communication channel interconnecting the sending and receiving stations forms a radio communication channel. A radio communication channel is defined upon a radio link formed upon a portion of the electromagnetic spectrum. In contrast, in a conventional wire line communication system, a physical connection is required to interconnect the sending and receiving stations to permit a wire line communication channel to be defined thereon. Increased communication mobility is inherently provided in a radio communication system, therefore, in contrast to communications in a conventional, wire line communication system.
Communication of data upon a radio communication channel is, however, particularly susceptible to distortion caused by non-ideal propagation characteristics of the radio communication channel. Data communicated upon communication channels defined in other communication systems are also susceptible to such distortion.
Non-ideal propagation characteristics include multi-path propagation of the data. Multi-path propagation of the data upon a radio, or other, communication channel introduces distortion upon the data as the data is communicated to the receiving station by way of a multiple number of paths. The data detected at the receiving station, as a result, is the combination of signal values of data communicated upon a plurality of different communication paths. Intersymbol interference and Rayleigh fading, for instance, causes distortion of the data. If not compensated for, such distortion prevents the accurate recovery of the transmitted data.
Various manners are used to facilitate recovery of the actually-transmitted data, i.e., the informational content thereof, responsive to analysis of the actually-received data, received at the receiving station. For instance, equalization operations performed by functional equalizer devices facilitate recovery of the informational content of the actually-transmitted data responsive to detection at the receiving station of the transmitted data. MLSE (maximum likelihood sequence estimation) techniques are sometimes utilized by equalizers to estimate values of the actually-transmitted data responsive to the detection of the received data.
The digital data that is communicated between the sending and receiving stations, in some communication systems, is selected from a constellation of allowable symbol values. Increases in the number of symbols contained in the symbol constellation permits higher capacities of communication due to the increased possible values of which the digital data may be formed. However, with the increase in the number of allowable symbols, i.e., a higher density constellation set, detection operations performed at a receiving station to recover the informational content of the transmitted data becomes more complex. And, sampling rates at which the received data is sampled also are increasing, further adding to the complexity required of equalizer operations.
Operations required of conventional equalizers to perform equalization operations upon digital data selected from high-density constellation sets and sampled at high data rates are, at present, unfeasible using the processing capacity of existing processing devices used at receiving stations, such as mobile stations operable in a cellular communication system.
Therefore, alternate equalization, and corresponding decoding, schemes are required to be implemented to permit the equalization, and decoding, operations to be performed upon data selected from higher-density constellation sets and sampled at higher sampling rates.
It is in light of this background information related to the communication of digital data that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus, and an associated method, by which to facilitate recovery of digital data communicated to a receiver upon a communication channel susceptible to distortion.
Through operation of an embodiment of the present invention, a manner is provided by which to perform MLSE (maximum likelihood sequence estimation) upon digital data received at the receiver.
Data is first applied to areliability indicia generator. The reliability indicia generator operates at a relatively low level of complexity through the use of a channel-shortening prefilter used in conjunction with an MLSE or other detector. Reliability information is generated with respect to possible values of the data, prior to its communication upon the non-ideal communication channel. Tthe reliability indicia generator is of a relatively low complexity, and the reliability information is produced quickly and without excessive processing requirements. The reliability information is then used at a trellis decoder of full, impulse response length and operable to select most-likely values of a sequence of the digital data. The number of nodes forming the trellis decoder is selected responsive to values of the reliability information. Nodes associated with data values, indicated by the reliability information, to be unlikely to occur are removed out of the trellis. By removing the node, or nodes, out of the trellis, the computational complexity of the sequence estimation as performed by the trellis decoder is reduced. By increasing the number of nodes that are reduced out of the trellis, a corresponding reduction in the computational complexity of the estimation computations is permitted.
In one aspect of the present invention, a manner is provided by which to perform MLSE (maximum likelihood sequence estimation) detection and decoding in which use of a conventional MLSE detection or decoding operation would be prohibitively complex, such as when a high-density constellation symbol set is utilized or a high sampling rate is used to sample received data. Reliability information is derived by a reliability indicia generator for each symbol, or bit, in a sequence of digital data upon which MLSE detection or decoding operations are to be performed. And, then a soft trellis is created. The soft trellis includes dynamically-sized trellis states. The size of the trellis is selectably reduced responsive to the reliability information that is first derived. The reliability information provides indications of whether a state-value is likely to have been the value of the actually-transmitted data that were assembled. A Viterbi algorithm is executed to perform the MLSE detection or decoding of the sequence of data. Because the number of calculations required to be performed pursuant to execution of the Viterbi algorithm to perform the maximum likelihood sequence estimation is reduced, processing complexity is reduced pursuant to operation of an embodiment of the present invention. Trellis-sizing is dynamically selected so that the required processing to perform the estimation operations is dependent upon the reliability information provided by thereliability indicia generator.
In another aspect of the present invention, multi-user detection in a dispersive channel in which a large number of symbols are used in a modulation alphabet is effectuable at a mobile station operable in a cellular, or other, radio communication system. The amount of turbo or iterative equalization and decoding complexity is significantly reduced, relative to conventional requirements, permitting such operations to be performed at a conventional DSP (digital signal processor) at complexity levels comparable to the use of conventional techniques utilized to estimate data values available in a communication system which utilizes a conventional-density constellation set.
In these and other aspects, therefore, apparatus, and an associated method, is provided for a receiver operable in a communication system to receive transmitted digital data. The transmitted digital data is transmitted to the receiver upon a channel susceptible to distortion. The transmitted digital data is of values at least representative of values of sending-station digital data. The values of the sending-station digital values are recovered responsive to detection at the receiver of the transmitted digital data. A reliability indicia generator is coupled to receive a sequence of the transmitted digital data. The reliability indicia generator forms reliability indicia associated with possible values of a corresponding sequence of the sending-station digital data. A trellis detector is coupled to the reliability indicia generator to receive indications of the reliability indications formed by thereliability indicia generator. The trellis detector is also coupled to receive the indications of the transmitted digital data. The trellis detector trellis decodes the sequence of the transmitted digital data to detect the values thereof. The trellis detector is of a node-size reduced by at least one trellis node responsive to values of the indications of the reliability indicia.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the invention, and the appended claims.